Prevalence of congenital myopathies in a representative pediatric united states population

Molecular mechanisms and therapeutic strategies for neuromuscular diseases

Neuromuscular diseases encompass a heterogeneous array of disorders characterized by varying onset ages, clinical presentations, severity, and progression. While these conditions can stem from acquired or inherited causes, this review specifically focuses on disorders arising from genetic abnormalities, excluding metabolic conditions. The pathogenic defect may primarily affect the anterior horn cells, the axonal or myelin component of peripheral nerves, the neuromuscular junction, or skeletal and/or cardiac muscles. While inherited neuromuscular disorders have been historically deemed not treatable, the advent of gene-based and molecular therapies is reshaping the treatment landscape for this group of condition. With the caveat that many products still fail to translate the positive results obtained in pre-clinical models to humans, both the technological development (e.g., implementation of tissue-specific vectors) as well as advances on the knowledge of pathogenetic mechanisms form a collective foundation for potentially curative approaches to these debilitating conditions. This review delineates the current panorama of therapies targeting the most prevalent forms of inherited neuromuscular diseases, emphasizing approved treatments and those already undergoing human testing, offering insights into the state-of-the-art interventions.

Rycal S48168 (ARM210) for RYR1-related myopathies: a phase one, open-label, dose-escalation trial

Background

RYR1-related myopathies (RYR1-RM) are caused by pathogenic variants in the RYR1 gene which encodes the type 1 ryanodine receptor (RyR1). RyR1 is the sarcoplasmic reticulum (SR) calcium release channel that mediates excitation-contraction coupling in skeletal muscle. RyR1 sub-conductance, SR calcium leak, reduced RyR1 expression, and oxidative stress often contribute to RYR1-RM pathogenesis. Loss of RyR1-calstabin1 association, SR calcium leak, and increased RyR1 open probability were observed in 17 RYR1-RM patient skeletal muscle biopsies and improved following ex vivo treatment with Rycal compounds. Thus, we initiated a first-in-patient trial of Rycal S48168 (ARM210) in ambulatory adults with genetically confirmed RYR1-RM.

Methods

Participants received 120 mg (n = 3) or 200 mg (n = 4) S48168 (ARM210) daily for 29 days. The primary endpoint was safety and tolerability. Exploratory endpoints included S48168 (ARM210) pharmacokinetics (PK), target engagement, motor function measure (MFM)-32, hand grip and pinch strength, timed functional tests, PROMIS fatigue scale, semi-quantitative physical exam strength measurements, and oxidative stress biomarkers. The trial was registered with clinicaltrials.gov (NCT04141670) and was conducted at the National Institutes of Health Clinical Center between October 28, 2019 and December 12, 2021.

Findings

S48168 (ARM210) was well-tolerated, did not cause any serious adverse events, and exhibited a dose-dependent PK profile. Three of four participants who received the 200 mg/day dose reported improvements in PROMIS-fatigue at 28 days post-dosing, and also demonstrated improved proximal muscle strength on physical examination.

Interpretation

S48168 (ARM210) demonstrated favorable safety, tolerability, and PK, in RYR1-RM affected individuals. Most participants who received 200 mg/day S48168 (ARM210) reported decreased fatigue, a key symptom of RYR1-RM. These results set the foundation for a randomized, double-blind, placebo-controlled proof of concept trial to determine efficacy of S48168 (ARM210) in RYR1-RM.

Funding

NINDS and NINR Intramural Research Programs, NIH Clinical Center Bench to Bedside Award (2017-551673), ARMGO Pharma Inc., and its development partner Les Laboratoires Servier.

Improving Diagnostic Precision: Phenotype-Driven Analysis Uncovers a Maternal Mosaicism in an Individual with RYR1-Congenital Myopathy

Congenital myopathies (CMs) are rare genetic disorders for which the diagnostic yield does not typically exceed 60% . We performed deep phenotyping, histopathological studies, clinical exome and trio genome sequencing and a phenotype-driven analysis of the genomic data, that led to the molecular diagnosis in a child with CM. We identified a heterozygous variant in RYR1 in the affected child, inherited from her asymptomatic mother. Given the alignment of the clinical and histopathological phenotype with RYR1-CM, we considered the potential existence of a missing second variant in trans in the proband, but also hypothesized that the variant might be mosaic in the mother, as subsequently demonstrated. Our study is an example of how heterozygous variants inherited from asymptomatic parents are frequently dismissed. When the genotype-phenotype correlation is strong, it is recommended to consider a parental mosaicism.

Marked Facial Weakness, Ptosis, and Hanging Jaw: A Case with RYR1 -Related Congenital Centronuclear Myopathy

Congenital myopathies are an expanding spectrum of neuromuscular disorders with early infantile or childhood onset hypotonia and slowly or nonprogressive skeletal muscle weakness. RYR1 -related myopathies are the most common and frequently diagnosed class of congenital myopathies. Malignant hyperthermia susceptibility and central core disease are autosomal dominant or de novo RYR1 disorder, whereas multiminicore, congenital fiber type disproportion and centronuclear myopathy are autosomal recessive RYR1 disorders. The presence of ptosis, ophthalmoparesis, facial, and proximal muscles weakness, with the presence of dusty cores and multiple internal nuclei on muscle biopsy are clues to the diagnosis. We describe an 18-year-old male, who presented with early infantile onset ptosis, ophthalmoplegia, myopathic facies, hanging lower jaw, and proximal muscle weakness confirmed as an RYR1 -related congenital centronuclear myopathy on genetic analysis and muscle biopsy.

Long-term Natural History of Pediatric Dominant and Recessive RYR1-Related Myopathy

BACKGROUND AND OBJECTIVES

RYR1-related myopathies are the most common congenital myopathies, but long-term natural history data are still scarce. We aim to describe the natural history of dominant and recessive RYR1-related myopathies.

METHODS

A cross-sectional and longitudinal retrospective data analysis of pediatric cases with RYR1-related myopathies seen between 1992-2019 in 2 large UK centers. Patients were identified, and data were collected from individual medical records.

RESULTS

Sixty-nine patients were included in the study, 63 in both cross-sectional and longitudinal studies and 6 in the cross-sectional analysis only. Onset ranged from birth to 7 years. Twenty-nine patients had an autosomal dominant RYR1-related myopathy, 31 recessive, 6 de novo dominant, and 3 uncertain inheritance. Median age at the first and last appointment was 4.0 and 10.8 years, respectively. Fifteen% of patients older than 2 years never walked (5 recessive, 4 de novo dominant, and 1 dominant patient) and 7% lost ambulation during follow-up. Scoliosis and spinal rigidity were present in 30% and 17% of patients, respectively. Respiratory involvement was observed in 22% of patients, and 12% needed ventilatory support from a median age of 7 years. Feeding difficulties were present in 30% of patients, and 57% of those needed gastrostomy or tube feeding. There were no anesthetic-induced malignant hyperthermia episodes reported in this cohort. We observed a higher prevalence of prenatal/neonatal features in recessive patients, in particular hypotonia and respiratory difficulties. Clinical presentation, respiratory outcomes, and feeding outcomes were consistently more severe at presentation and in the recessive group. Conversely, longitudinal analysis suggested a less progressive course for motor and respiratory function in recessive patients. Annual change in forced vital capacity was -0.2%/year in recessive vs -1.4%/year in dominant patients.

DISCUSSION

This clinical study provides long-term data on disease progression in RYR1-related myopathies that may inform management and provide essential milestones for future therapeutic interventions.

Abnormal myosin post-translational modifications and ATP turnover time associated with human congenital myopathy-related RYR1 mutations

AIM

Conditions related to mutations in the gene encoding the skeletal muscle ryanodine receptor 1 (RYR1) are genetic muscle disorders and include congenital myopathies with permanent weakness, as well as episodic phenotypes such as rhabdomyolysis/myalgia. Although RYR1 dysfunction is the primary mechanism in RYR1-related disorders, other downstream pathogenic events are less well understood and may include a secondary remodeling of major contractile proteins. Hence, in the present study, we aimed to investigate whether congenital myopathy-related RYR1 mutations alter the regulation of the most abundant contractile protein, myosin.

METHODS

We used skeletal muscle tissues from five patients with RYR1-related congenital myopathy and compared those with five controls and five patients with RYR1-related rhabdomyolysis/myalgia. We then defined post-translational modifications on myosin heavy chains (MyHCs) using LC/MS. In parallel, we determined myosin relaxed states using Mant-ATP chase experiments and performed molecular dynamics (MD) simulations.

RESULTS

LC/MS revealed two additional phosphorylations (Thr1309-P and Ser1362-P) and one acetylation (Lys1410-Ac) on the β/slow MyHC of patients with congenital myopathy. This method also identified six acetylations that were lacking on MyHC type IIa of these patients (Lys35-Ac, Lys663-Ac, Lys763-Ac, Lys1171-Ac, Lys1360-Ac, and Lys1733-Ac). MD simulations suggest that modifying myosin Ser1362 impacts the protein structure and dynamics. Finally, Mant-ATP chase experiments showed a faster ATP turnover time of myosin heads in the disordered-relaxed conformation.

CONCLUSIONS

Altogether, our results suggest that RYR1 mutations have secondary negative consequences on myosin structure and function, likely contributing to the congenital myopathic phenotype.

Protein Aggregates and Aggrephagy in Myopathies

A number of muscular disorders are hallmarked by the aggregation of misfolded proteins within muscle fibers. A specialized form of macroautophagy, termed aggrephagy, is designated to remove and degrade protein aggregates. This review aims to summarize what has been studied so far about the direct involvement of aggrephagy and the activation of the key players, among others, p62, NBR1, Alfy, Tollip, Optineurin, TAX1BP1 and CCT2 in muscular diseases. In the first part of the review, we describe the aggrephagy pathway with the involved proteins; then, we illustrate the muscular disorder histologically characterized by protein aggregates, highlighting the role of aggrephagy pathway abnormalities in these muscular disorders.

A novel RYR1 variant in an infant with a unique fetal presentation of central core disease

Ryanodine receptor type 1-related disorder (RYR1-RD) is the most common subgroup of congenital myopathies with a wide phenotypic spectrum ranging from mild hypotonia to lethal fetal akinesia. Genetic testing for myopathies is imperative as the diagnosis informs counseling regarding prognosis and recurrence risk, treatment options, monitoring, and clinical management. However, diagnostic challenges exist as current options are limited to clinical suspicion prompting testing including: single gene sequencing or familial variant testing, multi-gene panels, exome, genome sequencing, and invasive testing including muscle biopsy. The timing of diagnosis is of great importance due to the association of RYR1-RD with malignant hyperthermia (MH). MH is a hypermetabolic crisis that occurs secondary to excessive calcium release in muscles, leading to systemic effects that can progress to shock and death if unrecognized. Given the association of MH with pathogenic variants in RYR1, a diagnosis of RYR1-RD necessitates an awareness of medical team to avoid potentially triggering agents. We describe a case of a unique fetal presentation with bilateral diaphragmatic eventrations who had respiratory failure, dysmorphic facial features, and profound global hypotonia in the neonatal period. The diagnosis was made at several months of age, had direct implications on her clinical care related to anticipated need to long-term ventilator support, and ultimately death secondary an arrhythmia as a result of suspected MH. Our report reinforces the importance of having high suspicion for a genetic syndrome and pursuing early, rapid exome or genome sequencing as first line testing in critically ill neonatal intensive care unit patients and further evaluating the pathogenicity of a variant of uncertain significance in the setting of a myopathic phenotype.

Ryanodine receptor 1 related myasthenia like myopathy responsive to pyridostigmine

Disease causing variants in the Ryanodine receptor 1 (RYR1) gene are a common cause for congenital myopathy and for malignant hyperthermia susceptibility. We report a 17 year old boy with congenital muscle weakness progressing to a myasthenia like myopathy with muscle weakness, fatigability, ptosis, and ophthalmoplegia. Muscle biopsy showed predominance and atrophy of type 1 fibers. Whole-exome trio sequencing revealed three variants in the RYR1-gene in the patient: c.6721C > T,p.(Arg2241*) and c.2122G > A,p.(Asp708Asn) in cis position, and the c.325C > T,p.(Arg109Trp) variant in trans. Treatment with pyridostigmine improved symptoms. This case supports that a myasthenia like phenotype is part of the phenotypic spectrum of RYR1 related disorders, and that treatment with pyridostigmine can be beneficial for patients with this phenotype.

A review of major causative genes in congenital myopathies

In this review, we focus on congenital myopathies, which are a genetically heterogeneous group of hereditary muscle diseases with slow or minimal progression. They are mainly defined and classified according to pathological features, with the major subtypes being core myopathy (central core disease), nemaline myopathy, myotubular/centronuclear myopathy, and congenital fiber-type disproportion myopathy. Recent advances in molecular genetics, especially next-generation sequencing technology, have rapidly increased the number of known causative genes for congenital myopathies; however, most of the diseases related to the novel causative genes are extremely rare. There remains no cure for congenital myopathies. However, there have been recent promising findings that could inform the development of therapy for several types of congenital myopathies, including myotubular myopathy, which indicates the importance of prompt and correct diagnosis. This review discusses the major causative genes (NEB, ACTA1, ADSSL1, RYR1, SELENON, MTM1, DNM2, and TPM3) for each subtype of congenital myopathies and the relevant latest findings.

European Neuromuscular Centre consensus statement on anaesthesia in patients with neuromuscular disorders

BACKGROUND AND PURPOSE

Patients with neuromuscular conditions are at increased risk of suffering perioperative complications related to anaesthesia. There is currently little specific anaesthetic guidance concerning these patients. Here, we present the European Neuromuscular Centre (ENMC) consensus statement on anaesthesia in patients with neuromuscular disorders as formulated during the 259th ENMC Workshop on Anaesthesia in Neuromuscular Disorders.

METHODS

International experts in the field of (paediatric) anaesthesia, neurology, and genetics were invited to participate in the ENMC workshop. A literature search was conducted in PubMed and Embase, the main findings of which were disseminated to the participants and presented during the workshop. Depending on specific expertise, participants presented the existing evidence and their expert opinion concerning anaesthetic management in six specific groups of myopathies and neuromuscular junction disorders. The consensus statement was prepared according to the AGREE II (Appraisal of Guidelines for Research & Evaluation) reporting checklist. The level of evidence has been adapted according to the SIGN (Scottish Intercollegiate Guidelines Network) grading system. The final consensus statement was subjected to a modified Delphi process.

RESULTS

A set of general recommendations valid for the anaesthetic management of patients with neuromuscular disorders in general have been formulated. Specific recommendations were formulated for (i) neuromuscular junction disorders, (ii) muscle channelopathies (nondystrophic myotonia and periodic paralysis), (iii) myotonic dystrophy (types 1 and 2), (iv) muscular dystrophies, (v) congenital myopathies and congenital dystrophies, and (vi) mitochondrial and metabolic myopathies.

CONCLUSIONS

This ENMC consensus statement summarizes the most important considerations for planning and performing anaesthesia in patients with neuromuscular disorders.

Targeted transcript analysis in muscles from patients with genetically diverse congenital myopathies

Congenital myopathies are a group of early onset muscle diseases of variable severity often with characteristic muscle biopsy findings and involvement of specific muscle types. The clinical diagnosis of patients typically relies on histopathological findings and is confirmed by genetic analysis. The most commonly mutated genes encode proteins involved in skeletal muscle excitation–contraction coupling, calcium regulation, sarcomeric proteins and thin–thick filament interaction. However, mutations in genes encoding proteins involved in other physiological functions (for example mutations in SELENON and MTM1, which encode for ubiquitously expressed proteins of low tissue specificity) have also been identified. This intriguing observation indicates that the presence of a genetic mutation impacts the expression of other genes whose product is important for skeletal muscle function. The aim of the present investigation was to verify if there are common changes in transcript and microRNA expression in muscles from patients with genetically heterogeneous congenital myopathies, focusing on genes encoding proteins involved in excitation–contraction coupling and calcium homeostasis, sarcomeric proteins, transcription factors and epigenetic enzymes. Our results identify RYR1, ATPB2B and miRNA-22 as common transcripts whose expression is decreased in muscles from congenital myopathy patients. The resulting protein deficiency may contribute to the muscle weakness observed in these patients. This study also provides information regarding potential biomarkers for monitoring disease progression and response to pharmacological treatments in patients with congenital myopathies.

A case of giant dental calculus in a patient with centronuclear myopathy

INTRODUCTION

Centronuclear myopathy is a hereditary congenital muscle disease. It is characterized by generalized muscle hypotonia from early childhood, elongated cacial appearance, mandibular undergroth, and dental malposition. In this report, we discuss the clinical course and management of a patient with centronuclearmyopathy, who developed a giant dental calculus in the floor of the mouth and underwent surgical excision.

CASE REPORT

A 37-year-old Japanese man was referred to our hospital, and reported a swelling in the floor of the mouth. The patient affects centronuclear myopathy and has generalized muscle weakness. CT images showed a high-density area in the floor of the mouth measuring 35 × 28 × 20 mm. The lesion was clinically diagnosed as giant dental calculus, and surgically removed.

CONCLUSION

We have experienced a case of giant dental calculus in a patient with centronuclear myopathy. In dental treatment, we must consider generalized muscle weakness.

Management of patients susceptible to malignant hyperthermia: A surgeon's perspective

OBJECTIVES

Malignant hyperthermia (MH) susceptibility caries broad implications for the care of pediatric surgical patients. While precautions must often be taken for only a vague family history, two options exist to assess MH-susceptibility. We evaluate the use of MH precautions and susceptibility testing at a freestanding children's hospital.

METHODS

This single institution retrospective cohort study identified patients of any age who received general anesthetics utilizing MH precautions over a five-year period. The electronic medical record was further queried for patients diagnosed with MH. The indication for MH precautions and uses of susceptibility testing are assessed. Secondary outcomes included a diagnosis of bona fide MH.

RESULTS

A total of 125 patients received 174 anesthetics with MH precautions at a mean age of 114 months (0-363 months). Otolaryngology was the procedural service most frequently involved in the care of the cohort (n = 45; 26%). A reported personal or family history of MH (n = 102; 59%) was the most common indication for precautions, followed by muscular dystrophy (n = 29; 17%). No MH events occurred in the cohort and further review of ICD-9 and -10 diagnosis codes found no MH diagnoses. No study subjects received muscle biopsy and contracture testing and only 5 (4%) underwent genetic testing for genomic variants known to cause MH susceptibility. A case example is given to highlight the implications of a reported MH history.

CONCLUSION

Otolaryngologists should maintain a familiarity with the precautions necessary to manage patients at risk for MH and MH-like reactions. Without an accessible test to rule out susceptibility, surgeons must rely on a careful history to appropriately utilize precautions. An inappropriate label of "MH-susceptible" may result in decreased access to care and treatment delays.

Diagnostic yield of multi-gene panel for muscular dystrophies and other hereditary myopathies

Genetic testing is being considered the first-step in the investigation of hereditary myopathies. However, the performance of the different testing approaches is little known. The aims of the present study were to evaluate the diagnostic yield of a next-generation sequencing panel comprising 39 genes as the first-tier test for genetic myopathies diagnosis and to characterize clinical and molecular findings of families from southern Brazil. Fifty-one consecutive index cases with clinical suspicion of genetic myopathies were recruited from October 2014 to March 2018 in a cross-sectional study. The overall diagnostic yield of the next-generation sequencing panel was 52.9%, increasing to 60.8% when including cases with candidate variants. Multi-gene panel solved the diagnosis of 12/25 (48%) probands with limb-girdle muscular dystrophies, of 7/14 (50%) with congenital muscular diseases, and of 7/10 (70%) with muscular dystrophy with prominent joint contractures. The most frequent diagnosis for limb-girdle muscular dystrophies were LGMD2A/LGMD-R1-calpain3-related and LGMD2B/LGMD-R2-dysferlin-related; for congenital muscular diseases, RYR1-related-disorders; and for muscular dystrophy with prominent joint contractures, Emery-Dreifuss-muscular-dystrophy-type-1 and COL6A1-related-disorders. In summary, the customized next-generation sequencing panel when applied in the initial investigation of genetic myopathies results in high diagnostic yield, likely reducing patient's diagnostic odyssey and providing important information for genetic counseling and participation in disease-specific clinical trials.

Identification of ER/SR resident proteins as biomarkers for ER/SR calcium depletion in skeletal muscle cells

BACKGROUND

Aberrations to endoplasmic/sarcoplasmic reticulum (ER/SR) calcium concentration can result in the departure of endogenous proteins in a phenomenon termed exodosis. Redistribution of the ER/SR proteome can have deleterious effects to cell function and cell viability, often contributing to disease pathogenesis. Many proteins prone to exodosis reside in the ER/SR via an ER retention/retrieval sequence (ERS) and are involved in protein folding, protein modification, and protein trafficking. While the consequences of their extracellular presence have yet to be fully delineated, the proteins that have undergone exodosis may be useful for biomarker development. Skeletal muscle cells rely upon tightly coordinated ER/SR calcium release for muscle contractions, and perturbations to calcium homeostasis can result in myopathies. Ryanodine receptor type-1 (RYR1) is a calcium release channel located in the SR. Mutations to the RYR1 gene can compromise calcium homeostasis leading to a vast range of clinical phenotypes encompassing hypotonia, myalgia, respiratory insufficiency, ophthalmoplegia, fatigue and malignant hyperthermia (MH). There are currently no FDA approved treatments for RYR1-related myopathies (RYR1-RM).

RESULTS

Here we examine the exodosis profile of skeletal muscle cells following ER/SR calcium depletion. Proteomic analysis identified 4,465 extracellular proteins following ER/SR calcium depletion with 1,280 proteins significantly different than vehicle. A total of 54 ERS proteins were identified and 33 ERS proteins significantly increased following ER/SR calcium depletion. Specifically, ERS protein, mesencephalic astrocyte-derived neurotrophic factor (MANF), was elevated following calcium depletion, making it a potential biomarker candidate for human samples. Despite no significant elevation of MANF in plasma levels among healthy volunteers and RYR1-RM individuals, MANF plasma levels positively correlated with age in RYR1-RM individuals, presenting a potential biomarker of disease progression. Selenoprotein N (SEPN1) was also detected only in extracellular samples following ER/SR calcium depletion. This protein is integral to calcium handling and SEPN1 variants have a causal role in SEPN1-related myopathies (SEPN1-RM). Extracellular presence of ER/SR membrane proteins may provide new insight into proteomic alterations extending beyond ERS proteins. Pre-treatment of skeletal muscle cells with bromocriptine, an FDA approved drug recently found to have anti-exodosis effects, curbed exodosis of ER/SR resident proteins.

CONCLUSION

Changes to the extracellular content caused by intracellular calcium dysregulation presents an opportunity for biomarker development and drug discovery.

Structural Insight Into Ryanodine Receptor Channelopathies

The ryanodine receptors (RyRs) are large cation-selective ligand-gated channels that are expressed in the sarcoplasmic reticulum (SR) membrane. They mediate the controlled release of Ca²⁺ from SR and play an important role in many cellular processes. The mutations in RyRs are associated with several skeletal muscle and cardiac conditions, including malignant hyperthermia (MH), central core disease (CCD), catecholaminergic polymorphic ventricular tachycardia (CPVT), and arrhythmogenic right ventricular dysplasia (ARVD). Recent breakthroughs in structural biology including cryo-electron microscopy (EM) and X-ray crystallography allowed the determination of a number of near-atomic structures of RyRs, including wildtype and mutant structures as well as the structures in complex with different modulating molecules. This allows us to comprehend the physiological gating and regulatory mechanisms of RyRs and the underlying pathological mechanisms of the disease-causing mutations. In this review, based on the insights gained from the available high-resolution structures of RyRs, we address several questions: 1) what are the gating mechanisms of different RyR isoforms; 2) how RyRs are regulated by multiple channel modulators, including ions, small molecules, and regulatory proteins; 3) how do disease-causing mutations affect the structure and function of RyRs; 4) how can these structural information aid in the diagnosis of the related diseases and the development of pharmacological therapies.

Characterization of a novel zebrafish model of SPEG-related centronuclear myopathy

Centronuclear myopathy (CNM) is a congenital neuromuscular disorder caused by pathogenic variation in genes associated with membrane trafficking and excitation–contraction coupling (ECC). Bi-allelic autosomal-recessive mutations in striated muscle enriched protein kinase (SPEG) account for a subset of CNM patients. Previous research has been limited by the perinatal lethality of constitutive Speg knockout mice. Thus, the precise biological role of SPEG in developing skeletal muscle remains unknown. To address this issue, we generated zebrafish spega, spegb and spega;spegb (speg-DKO) mutant lines. We demonstrated that speg-DKO zebrafish faithfully recapitulate multiple phenotypes associated with CNM, including disruption of the ECC machinery, dysregulation of calcium homeostasis during ECC and impairment of muscle performance. Taking advantage of zebrafish models of multiple CNM genetic subtypes, we compared novel and known disease markers in speg-DKO with mtm1-KO and DNM2-S619L transgenic zebrafish. We observed Desmin accumulation common to all CNM subtypes, and Dnm2 upregulation in muscle of both speg-DKO and mtm1-KO zebrafish. In all, we establish a new model of SPEG-related CNM, and identify abnormalities in this model suitable for defining disease pathomechanisms and evaluating potential therapies.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: We created a novel zebrafish Speg mutant model of centronuclear myopathy that recapitulates key features of the human disorder and provides insight into pathomechanisms of the disease.

Facial Asymmetry: A Narrative Review of the Most Common Neurological Causes

Symmetry of the face is one of the most important features for the perception of attractiveness. Asymmetry of the face means that the right and left sides of the face are not identical. Subclinical facial asymmetry is very common in the general population. Most people have some slight facial asymmetry, and this is the normal biological situation in humans. Abnormalities from soft tissue, dental, and skeletal elements lead to facial asymmetry. Asymmetry has many causal factors, and its aetiology includes both congenital and acquired conditions. Neurological facial asymmetry is scarcely addressed in the dental literature. In this narrative review, we focus on the most common neurological causes of facial asymmetry. From a neurological point of view, facial asymmetry can result from disturbances of the cranial nerves, developmental disorders, or myopathies. In the broad range of differential diagnostics of facial asymmetry, neurological abnormalities should be taken into consideration. The treatment must be related to the underlying cause. Complete knowledge of the aetiological factors and the character of the asymmetry plays a crucial role in formulating a treatment plan.

Corrigendum: A Systematic Review and Meta-Analysis of the Prevalence of Congenital Myopathy

[This corrects the article DOI: 10.3389/fneur.2021.761636.].

RyR1-related myopathy mutations in ATP and calcium binding sites impair channel regulation

The type 1 ryanodine receptor (RyR1) is an intracellular calcium (Ca²⁺) release channel on the sarcoplasmic/endoplasmic reticulum that is required for skeletal muscle contraction. RyR1 channel activity is modulated by ligands, including the activators Ca²⁺ and ATP. Patients with inherited mutations in RyR1 may exhibit muscle weakness as part of a heterogeneous, complex disorder known as RYR1-related myopathy (RYR1-RM) or more recently termed RYR1-related disorders (RYR1-RD). Guided by high-resolution structures of skeletal muscle RyR1, obtained using cryogenic electron microscopy, we introduced mutations into putative Ca²⁺ and ATP binding sites and studied the function of the resulting mutant channels. These mutations confirmed the functional significance of the Ca²⁺ and ATP binding sites identified by structural studies based on the effects on channel regulation. Under normal conditions, Ca²⁺ activates RyR1 at low concentrations (µM) and inhibits it at high concentrations (mM). Mutations in the Ca²⁺-binding site impaired both activating and inhibitory regulation of the channel, suggesting a single site for both high and low affinity Ca²⁺-dependent regulation of RyR1 function. Mutation of residues that interact with the adenine ring of ATP abrogated ATP binding to the channel, whereas mutating residues that interact with the triphosphate tail only affected the degree of activation. In addition, patients with mutations at the Ca²⁺ or ATP binding sites suffer from muscle weakness, therefore impaired RyR1 channel regulation by either Ca²⁺ or ATP may contribute to the pathophysiology of RYR1-RM in some patients.

A Systematic Review and Meta-Analysis of the Prevalence of Congenital Myopathy

Background: Congenital myopathy constitutes a heterogeneous group of orphan diseases that are mainly classified on the basis of muscle biopsy findings. This study aims to estimate the prevalence of congenital myopathy through a systematic review and meta-analysis of the literature.

Methods: The PubMed, MEDLINE, Web of Science, and Cochrane Library databases were searched for original research articles published in English prior to July 30, 2021. The quality of the included studies was assessed by a checklist adapted from STrengthening the Reporting of OBservational studies in Epidemiology (STROBE). To derive the pooled epidemiological prevalence estimates, a meta-analysis was performed using the random effects model. Heterogeneity was assessed using the Cochrane Q statistic as well as the I² statistic.

Results: A total of 11 studies were included in the systematic review and meta-analysis. Of the 11 studies included, 10 (90.9%) were considered medium-quality, one (9.1%) was considered low-quality, and no study was assessed as having a high overall quality. The pooled prevalence of congenital myopathy in the all-age population was 1.62 (95% CI, 1.13–2.11) per 100,000, while the prevalence in the child population was 2.76 (95% CI, 1.34–4.18) per 100,000. In the pediatric population, the prevalence among males was 2.92 (95% CI, −1.70 to 7.55) per 100,000, while the prevalence among females was 2.47 (95% CI, −1.67 to 6.61) per 100,000. The prevalence estimates of the all-age population per 100,000 were 0.20 (95% CI 0.10–0.35) for nemaline myopathy, 0.37 (95% CI 0.21–0.53) for core myopathy, 0.08 (95% CI −0.01 to 0.18) for centronuclear myopathy, 0.23 (95% CI 0.04–0.42) for congenital fiber-type disproportion myopathy, and 0.34 (95% CI, 0.24–0.44) for unspecified congenital myopathies. In addition, the prevalence estimates of the pediatric population per 100,000 were 0.22 (95% CI 0.03–0.40) for nemaline myopathy, 0.46 (95% CI 0.03–0.90) for core myopathy, 0.44 (95% CI 0.03–0.84) for centronuclear myopathy, 0.25 (95% CI −0.05 to 0.54) for congenital fiber-type disproportion myopathy, and 2.63 (95% CI 1.64–3.62) for unspecified congenital myopathies.

Conclusions: Accurate estimates of the prevalence of congenital myopathy are fundamental to supporting public health decision-making. The high heterogeneity and the lack of high-quality studies highlight the need to conduct higher-quality studies on orphan diseases.

Myocardial and Arrhythmic Spectrum of Neuromuscular Disorders in Children

Neuromuscular disorders (NMDs) are highly heterogenous from both an etiological and clinical point of view. Their signs and symptoms are often multisystemic, with frequent cardiac involvement. In fact, childhood onset forms can predispose a person to various progressive cardiac abnormalities including cardiomyopathies (CMPs), valvulopathies, atrioventricular conduction defects (AVCD), supraventricular tachycardia (SVT) and ventricular arrhythmias (VA). In this review, we selected and described five specific NMDs: Friedreich’s Ataxia (FRDA), congenital and childhood forms of Myotonic Dystrophy type 1 (DM1), Kearns Sayre Syndrome (KSS), Ryanodine receptor type 1-related myopathies (RYR1-RM) and Laminopathies. These changes are widely investigated in adults but less researched in children. We focused on these specific topics due their relative frequency and their potential unexpected cardiac manifestations in children. Moreover these conditions present different inheritance patterns and mechanisms of action. We decided not to discuss Duchenne and Becker muscular dystrophies due to extensive work regarding the cardiac aspects in children. For each described NMD, we focused on the possible cardiac manifestations such as different types of CMPs (dilated-DCM, hypertrophic-HCM, restrictive-RCM or left ventricular non compaction-LVNC), structural heart abnormalities (including valvulopathies), and progressive heart rhythm changes (AVCD, SVT, VA). We describe the current management strategies for these conditions. We underline the importance, especially for children, of a serial multidisciplinary personalized approach and the need for periodic surveillance by a dedicated heart team. This is largely due to the fact that in children, the diagnosis of certain NMDs might be overlooked and the cardiac aspect can provide signs of their presence even prior to overt neurological diagnosis.

Therapies for RYR1-Related Myopathies: Where We Stand and the Perspectives

RyR1-related myopathies are a family of genetic neuromuscular diseases due to mutations in the RYR1 gene. No treatment exists for any of these myopathies today, which could change in the coming years with the growing number of studies dedicated to the pre-clinical assessment of various approaches, from pharmacological to gene therapy strategies, using the numerous models developed up to now. In addition, the first clinical trials for these rare diseases have just been completed or are being launched. We review the most recent results obtained for the treatment of RyR1-related myopathies, and, in view of the progress in therapeutic development for other myopathies, we discuss the possible future therapeutic perspectives for RyR1-related myopathies.

Genetic therapy for congenital myopathies

PURPOSE OF REVIEW

There has been an explosion of advancement in the field of genetic therapies. The first gene-based treatments are now in clinical practice, with several additional therapeutic programs in various stages of development. Novel technologies are being developed that will further advance the breadth and success of genetic medicine.Congenital myopathies are an important group of neuromuscular disorders defined by structural changes in the muscle and characterized by severe clinical symptoms caused by muscle weakness. At present, there are no approved drug therapies for any subtype of congenital myopathy.In this review, we present an overview of genetic therapies and discuss their application to congenital myopathies.

RECENT FINDINGS

Several candidate therapeutics for congenital myopathies are in the development pipeline, including ones in clinical trial. These include genetic medicines such as gene replacement therapy and antisense oligonucleotide-based gene knockdown. We highlight the programs related to genetic medicine, and also discuss congenital myopathy subtypes where genetic therapy could be applied.

SUMMARY

Genetic therapies are ushering in an era of precision medicine for neurological diseases. Congenital myopathies are conditions ideally suited for genetic medicine approaches, and the first such therapies will hopefully soon be reaching congenital myopathy patients.

New diagnostic and therapeutic modalities in neuromuscular disorders in children

Pediatric neuromuscular disorders are a diverse group of conditions that affect how muscle and nerve function. They involve the motor neurons, nerves, neuromuscular junction and muscles. Pathology of any of these regions leads to the inability to perform voluntary movements. Over time, the natural progression of most of these disorders is followed by significant disability, and at the most extreme, almost complete paralysis and death secondary to complications. Diagnostic measures for pediatric neuromuscular disorders, like that of most medical conditions, relies heavily on clinical presentation, history and a detailed physical examination. Primary additional diagnostic measures have included serum creatine kinase (CK) levels, electromyography (EMG), nerve conduction studies (NCS) and muscle or nerve biopsies, which has historically been the gold standard. In the last several decades less invasive testing has become more common such as muscle magnetic resonance imaging (MRI) and genetic testing.  The advances of molecular genetics, such as next generation sequencing (NGS) which includes whole-exome sequencing (WES) and whole-genome sequencing (WGS), enable clinicians to pinpoint more accurately exact gene mutations. The advent of genetic testing enhances personalized medicine. The field of pediatric neuromuscular disorders is also undergoing a remarkable evolution in therapeutic modalities including novel targeted therapies such as exon skipping/inclusion and gene replacement therapies. This is a review of the initial approach to suspected neuromuscular disorders in children as well as up to date diagnostic and therapeutic modalities for the most common pediatric neuromuscular disorders. As the world enters the new decade, there are encouraging therapeutic results. However, there remain key challenges to these modalities including limitations in its applicability, optimization for delivery of gene replacement therapies and in its effectiveness.

Understanding Symptoms in RYR1-Related Myopathies: A Mixed-Methods Analysis Based on Participants' Experience

BACKGROUND

In rare diseases such as ryanodine receptor 1-related myopathies (RYR1-RM), health-related quality of life (HRQoL) measures are critically important so clinicians and researchers can better understand what symptoms are most important to participants, with the ultimate goal of finding tangible solutions for them.

OBJECTIVES

The main objective of this study was to characterize symptoms in individuals with RYR1-RM to inform future research. A secondary objective of this study was to analyze positive and negative sentiments regarding symptoms and treatment effects post N-acetylcysteine (NAC) administration in individuals with RYR1-RM.

METHODS

The study used a mixed-methods design applying methodological triangulation. Qualitative data were collected via semi-structured interviews at three visits to characterize symptoms in individuals with RYR1-RM and to analyze treatment effects. Qualitative data were then transformed into quantitative results to measure the frequency with which each symptom was mentioned by participants.

RESULTS

A total of 12 symptoms were identified as areas of interest to participants with RYR1-RM, highlighting fatigue and weakness as key symptoms. Data transformation categorized more than 1000 citations, reporting a greater number of positive comments for postintervention interviews than for baseline and preintervention visits and that NAC group participants stated more positive comments regarding treatment effect than did the placebo group.

CONCLUSIONS

We present a comprehensive characterization of symptoms in RYR1-RM and how those symptoms influence HRQoL. Furthermore, the introduction of mixed methods may be a valuable way to better understand patient-centered data in rare diseases to support affected individuals in coping with their symptoms.

Biallelic loss-of-function HACD1 variants are a bona fide cause of congenital myopathy

Congenital myopathies include a wide range of genetically determined disorders characterized by muscle weakness that usually manifest shortly after birth. To date, two different homozygous loss-of-function variants in the HACD1 gene have been reported to cause congenital myopathy. We identified three patients manifesting with neonatal-onset generalized muscle weakness and motor delay that carried three novel homozygous likely pathogenic HACD1 variants. The two of these changes (c.373_375+2delGAGGT and c.785-1G>T) were predicted to introduce splice site alterations, while one is a nonsense change (c.458G>A). The clinical presentation of our and the previously reported patients was comparable, including the temporally progressive improvement that seems to be characteristic of HACD1-related myopathy. Our findings conclusively confirm the implication of HACD1 in the pathogenesis of congenital myopathies, corroborate the main phenotypic features, and further define the genotypic spectrum of this genetic form of myopathy. Importantly, the genetic diagnosis of HACD1-related myopathy bears impactful prognostic value.

Myocardial strain assessment by 2D speckle-tracking echocardiography in patients with congenital myopathy

Background:

Congenital myopathies (CMs) are a group of rare genetic muscle disorders. Cardiac involvement can be seen in these patients. We aimed to evaluate the myocardial strain parameters by 2D speckle-tracking echocardiography (STE) in patients with CM.

Materials and Methods:

Twenty-four patients with CM whose diagnosis was confirmed by genetic analysis or muscle biopsy were included in the study, and 48 patients were involved as a control group. Left ventricular ejection fraction (LVEF%) was calculated by biplane Simpson method, and myocardial strain analysis was performed by 2D STE.

Results:

The median age of the study population was 26 (19–35 interquartile range [IQR]) and 43 (60%) were women. In the analysis performed after the exclusion of two patients with multiminicore disease (MMD) who developed heart failure, although mild, LVEF% (62 [60–65 IQR] vs. 64 [63–66 IQR], P = 0.008) and right ventricular global longitudinal strain (RVGLS) were significantly lower in the CM group (−21.8 [−19.7, −24.9 IQR] vs. −23.9 [−22.4, −25.6 IQR], P = 0.0017). Left ventricular global longitudinal strain (LVGLS) was observed similarly in both groups (−19.9 [−18.7, −20.7 IQR] vs. −20.5 [−19.3, −21.9 IQR], P = 0.069). LVEF% (33 and 46), LVGLS (−7.5 and −10.7), and RVGLS (−14.9 and −16.1) values were low in two siblings with MMD.

Conclusion:

Although LVEF% and RVGLS were significantly lower in the CM group, LVGLS was similar. The decrease in RVGLS and LVEF% was mild, and heart failure was not observed in any patient except MMD patients who were not included in the analysis.

Pathogenic Variants in the Myosin Chaperone UNC-45B Cause Progressive Myopathy with Eccentric Cores

The myosin-directed chaperone UNC-45B is essential for sarcomeric organization and muscle function from Caenorhabditis elegans to humans. The pathological impact of UNC-45B in muscle disease remained elusive. We report ten individuals with bi-allelic variants in UNC45B who exhibit childhood-onset progressive muscle weakness. We identified a common UNC45B variant that acts as a complex hypomorph splice variant. Purified UNC-45B mutants showed changes in folding and solubility. In situ localization studies further demonstrated reduced expression of mutant UNC-45B in muscle combined with abnormal localization away from the A-band towards the Z-disk of the sarcomere. The physiological relevance of these observations was investigated in C. elegans by transgenic expression of conserved UNC-45 missense variants, which showed impaired myosin binding for one and defective muscle function for three. Together, our results demonstrate that UNC-45B impairment manifests as a chaperonopathy with progressive muscle pathology, which discovers the previously unknown conserved role of UNC-45B in myofibrillar organization.

Core myopathies - a short review

Congenital myopathies represent a clinically and genetically heterogeneous group of early-onset neuromuscular diseases with characteristic, but not always specific, histopathological features, often presenting with stable and/or slowly progressive truncal and proximal weakness. It is often not possible to have a diagnosis on clinical ground alone. Additional extraocular, respiratory, distal involvement, scoliosis, and distal laxity may provide clues. The "core myopathies" collectively represent the most common form of congenital myopathies, and the name pathologically corresponds to histochemical appearance of focally reduced oxidative enzyme activity and myofibrillar changes on ultrastructural studies. Because of the clinical, pathological, and molecular overlaps, central core disease and multiminicore disease will be discussed together.

From Mice to Humans: An Overview of the Potentials and Limitations of Current Transgenic Mouse Models of Major Muscular Dystrophies and Congenital Myopathies

Muscular dystrophies are a group of more than 160 different human neuromuscular disorders characterized by a progressive deterioration of muscle mass and strength. The causes, symptoms, age of onset, severity, and progression vary depending on the exact time point of diagnosis and the entity. Congenital myopathies are rare muscle diseases mostly present at birth that result from genetic defects. There are no known cures for congenital myopathies; however, recent advances in gene therapy are promising tools in providing treatment. This review gives an overview of the mouse models used to investigate the most common muscular dystrophies and congenital myopathies with emphasis on their potentials and limitations in respect to human applications.

Ryanodine receptor 1-related disorders: an historical perspective and proposal for a unified nomenclature

The RYR1 gene, which encodes the sarcoplasmic reticulum calcium release channel or type 1 ryanodine receptor (RyR1) of skeletal muscle, was sequenced in 1988 and RYR1 variations that impair calcium homeostasis and increase susceptibility to malignant hyperthermia were first identified in 1991. Since then, RYR1-related myopathies (RYR1-RM) have been described as rare, histopathologically and clinically heterogeneous, and slowly progressive neuromuscular disorders. RYR1 variants can lead to dysfunctional RyR1-mediated calcium release, malignant hyperthermia susceptibility, elevated oxidative stress, deleterious post-translational modifications, and decreased RyR1 expression. RYR1-RM-affected individuals can present with delayed motor milestones, contractures, scoliosis, ophthalmoplegia, and respiratory insufficiency.

Historically, RYR1-RM-affected individuals were diagnosed based on morphologic features observed in muscle biopsies including central cores, cores and rods, central nuclei, fiber type disproportion, and multi-minicores. However, these histopathologic features are not always specific to RYR1-RM and often change over time. As additional phenotypes were associated with RYR1 variations (including King-Denborough syndrome, exercise-induced rhabdomyolysis, lethal multiple pterygium syndrome, adult-onset distal myopathy, atypical periodic paralysis with or without myalgia, mild calf-predominant myopathy, and dusty core disease) the overlap among diagnostic categories is ever increasing. With the continuing emergence of new clinical subtypes along the RYR1 disease spectrum and reports of adult-onset phenotypes, nuanced nomenclatures have been reported (RYR1- [related, related congenital, congenital] myopathies). In this narrative review, we provide historical highlights of RYR1 research, accounts of the main diagnostic disease subtypes and propose RYR1-related disorders (RYR1-RD) as a unified nomenclature to describe this complex and evolving disease spectrum.

Intracellular calcium leak as a therapeutic target for RYR1-related myopathies

RYR1 encodes the type 1 ryanodine receptor, an intracellular calcium release channel (RyR1) on the skeletal muscle sarcoplasmic reticulum (SR). Pathogenic RYR1 variations can destabilize RyR1 leading to calcium leak causing oxidative overload and myopathy. However, the effect of RyR1 leak has not been established in individuals with RYR1-related myopathies (RYR1-RM), a broad spectrum of rare neuromuscular disorders. We sought to determine whether RYR1-RM affected individuals exhibit pathologic, leaky RyR1 and whether variant location in the channel structure can predict pathogenicity. Skeletal muscle biopsies were obtained from 17 individuals with RYR1-RM. Mutant RyR1 from these individuals exhibited pathologic SR calcium leak and increased activity of calcium-activated proteases. The increased calcium leak and protease activity were normalized by ex-vivo treatment with S107, a RyR stabilizing Rycal molecule. Using the cryo-EM structure of RyR1 and a new dataset of > 2200 suspected RYR1-RM affected individuals we developed a method for assigning pathogenicity probabilities to RYR1 variants based on 3D co-localization of known pathogenic variants. This study provides the rationale for a clinical trial testing Rycals in RYR1-RM affected individuals and introduces a predictive tool for investigating the pathogenicity of RYR1 variants of uncertain significance.

Mixed methods analysis of Health-Related Quality of Life in ambulant individuals affected with RYR1-related myopathies pre-post-N-acetylcysteine therapy

PURPOSE

To characterize Health-Related Quality of Life (HRQoL) in ambulant individuals with RYR1-RM and to determine if a qualitative PRO tool (subjective self-assessment) complements PROMIS and Neuro-QoL scales to detect changes in HRQoL in ambulant individuals with RYR1-RM post N-acetylcysteine (NAC) treatment.

METHODS

The study used a mixed methods research (MMR) design applying methodological triangulation. Qualitative data were collected via semi-structured interviews using open-ended questions. Quantitative data were gathered through PROMIS and Neuro-QoL instruments. Additionally, qualitative data were transformed into quantitative data for subjective self-assessment and frequency analyses.

RESULTS

Qualitative results identified five domains and 33 subdomains as areas of interest. The most valuable were the importance of social impacts, the development of several coping strategies, both physical and psychological, and the identification of fatigue and weakness as key symptoms. Data transformation then categorized more than 3100 citations on frequency analyses, globally and by domain, visit, and participant. Regarding quantitative results, there was no clear evidence that any of the three PRO tools captured positive changes as a result of NAC treatment.

CONCLUSION

Qualitative results showed a comprehensive characterization of HRQoL in this population based on a symptom/patient-centered approach. These findings will inform future studies. Furthermore, given the similar findings across our multiple methods and endpoints, the introduction of MMR may be a valuable, complementary approach to clinical trials. MMR may be especially useful to incorporate in order to address and follow the FDA's guidance and prioritization on the inclusion of affected individuals' perspectives in clinical trials.

Update on Congenital Myopathies in Adulthood

Congenital myopathies (CMs) constitute a group of heterogenous rare inherited muscle diseases with different incidences. They are traditionally grouped based on characteristic histopathological findings revealed on muscle biopsy. In recent decades, the ever-increasing application of modern genetic technologies has not just improved our understanding of their pathophysiology, but also expanded their phenotypic spectrum and contributed to a more genetically based approach for their classification. Later onset forms of CMs are increasingly recognised. They are often considered milder with slower progression, variable clinical presentations and different modes of inheritance. We reviewed the key features and genetic basis of late onset CMs with a special emphasis on those forms that may first manifest in adulthood.

Preclinical model systems of ryanodine receptor 1-related myopathies and malignant hyperthermia: a comprehensive scoping review of works published 1990-2019

BACKGROUND

Pathogenic variations in the gene encoding the skeletal muscle ryanodine receptor (RyR1) are associated with malignant hyperthermia (MH) susceptibility, a life-threatening hypermetabolic condition and RYR1-related myopathies (RYR1-RM), a spectrum of rare neuromuscular disorders. In RYR1-RM, intracellular calcium dysregulation, post-translational modifications, and decreased protein expression lead to a heterogenous clinical presentation including proximal muscle weakness, contractures, scoliosis, respiratory insufficiency, and ophthalmoplegia. Preclinical model systems of RYR1-RM and MH have been developed to better understand underlying pathomechanisms and test potential therapeutics.

METHODS

We conducted a comprehensive scoping review of scientific literature pertaining to RYR1-RM and MH preclinical model systems in accordance with the PRISMA Scoping Reviews Checklist and the framework proposed by Arksey and O'Malley. Two major electronic databases (PubMed and EMBASE) were searched without language restriction for articles and abstracts published between January 1, 1990 and July 3, 2019.

RESULTS

Our search yielded 5049 publications from which 262 were included in this review. A majority of variants tested in RYR1 preclinical models were localized to established MH/central core disease (MH/CCD) hot spots. A total of 250 unique RYR1 variations were reported in human/rodent/porcine models with 95% being missense substitutions. The most frequently reported RYR1 variant was R614C/R615C (human/porcine total n = 39), followed by Y523S/Y524S (rabbit/mouse total n = 30), I4898T/I4897T/I4895T (human/rabbit/mouse total n = 20), and R163C/R165C (human/mouse total n = 18). The dyspedic mouse was utilized by 47% of publications in the rodent category and its RyR1-null (1B5) myotubes were transfected in 23% of publications in the cellular model category. In studies of transfected HEK-293 cells, 57% of RYR1 variations affected the RyR1 channel and activation core domain. A total of 15 RYR1 mutant mouse strains were identified of which ten were heterozygous, three were compound heterozygous, and a further two were knockout. Porcine, avian, zebrafish, C. elegans, canine, equine, and drosophila model systems were also reported.

CONCLUSIONS

Over the past 30 years, there were 262 publications on MH and RYR1-RM preclinical model systems featuring more than 200 unique RYR1 variations tested in a broad range of species. Findings from these studies have set the foundation for therapeutic development for MH and RYR1-RM.

Mouse model of severe recessive RYR1-related myopathy

Ryanodine receptor type I (RYR1)-related myopathies (RYR1 RM) are a clinically and histopathologically heterogeneous group of conditions that represent the most common subtype of childhood onset non-dystrophic muscle disorders. There are no treatments for this severe group of diseases. A major barrier to therapy development is the lack of an animal model that mirrors the clinical severity of pediatric cases of the disease. To address this, we used CRISPR/Cas9 gene editing to generate a novel recessive mouse model of RYR1 RM. This mouse (Ryr1TM/Indel) possesses a patient-relevant point mutation (T4706M) engineered into 1 allele and a 16 base pair frameshift deletion engineered into the second allele. Ryr1TM/Indel mice exhibit an overt phenotype beginning at 14 days of age that consists of reduced body/muscle mass and myofibre hypotrophy. Ryr1TM/Indel mice become progressively inactive from that point onward and die at a median age of 42 days. Histopathological assessment shows myofibre hypotrophy, increased central nuclei and decreased triad number but no clear evidence of metabolic cores. Biochemical analysis reveals a marked decrease in RYR1 protein levels (20% of normal) as compared to only a 50% decrease in transcript. Functional studies at end stage show significantly reduced electrically evoked Ca2+ release and force production. In summary, Ryr1TM/Indel mice exhibit a post-natal lethal recessive form of RYR1 RM that pheno-copies the severe congenital clinical presentation seen in a subgroup of RYR1 RM children. Thus, Ryr1TM/Indel mice represent a powerful model for both establishing the pathomechanisms of recessive RYR1 RM and pre-clinical testing of therapies for efficacy.

Exercise Training as Part of Musculoskeletal Management for Congenital Myopathy: Where Are We Now?

Congenital myopathy is a heterogeneous group of muscle disorders characterized by muscle weakness and hypotonia. This condition is associated with a range of skeletal, respiratory, and ophthalmologic complications and requires a multidisciplinary therapeutic approach aimed at maximizing the function and independence of patients. One promising direction for therapeutic intervention is physical exercise rehabilitation, given its demonstrated ability to promote muscle and bone health of patients with a variety of neuromuscular conditions. However, there are few data to assist health care professionals identify the optimal physical activity levels and exercise type, including the intensity, frequency, and duration. This lack of empirical evidence is particularly problematic given the fact that inappropriate exercise modes can potentially cause muscle damage in patients with congenital myopathy. In this article, we discuss the rationale behind the incorporation of two types of physical exercises, strength and aerobic training, into the clinical care of patients with congenital myopathy. Given the paucity of literature on the management of congenital myopathy, we review the results of published research on the treatment of both congenital myopathy and other neuromuscular diseases that could provide helpful insights into the physical rehabilitation of patients with congenital myopathy. We also discuss the potential benefits of vibration therapy, which has been studied in patients with other neuromuscular disorders over the last two decades. We conclude by proposing directions for future research on physical rehabilitation of patients with congenital myopathy.

Randomized controlled trial of N-acetylcysteine therapy for RYR1-related myopathies

Objective

To investigate the efficacy of N-acetylcysteine (NAC) for decreasing elevated oxidative stress and increasing physical endurance in individuals with ryanodine receptor 1-related myopathies (RYR1-RM).

Methods

In this 6-month natural history assessment (n = 37) followed by a randomized, double-blinded, placebo-controlled trial, 33 eligible participants were block-randomized (1:1) to receive NAC (n = 16) or placebo (n = 17), orally for 6 months (adult dose 2,700 mg/d; pediatric dose 30 mg/kg/d). The primary endpoint was urine 15-F2t isoprostane concentration and the clinically meaningful co-primary endpoint was 6-minute walk test (6MWT) distance.

Results

When compared to the general population, participants had elevated baseline 15-F2t isoprostane concentrations and most had a decreased 6MWT distance (mean ± SD 3.2 ± 1.5 vs 1.1 ± 1.7 ng/mg creatinine and 468 ± 134 vs 600 ± 58 m, respectively, both p < 0.001). 15-F2t isoprostane concentration and 6MWT distance did not change over the 6-month natural history assessment (p = 0.98 and p = 0.61, respectively). NAC treatment did not improve 15-F2t isoprostane concentration (least squares means difference 0.1 [95% confidence interval [CI] −1.4 to 1.6] ng/mg creatinine, p = 0.88) or 6MWT distance (least squares means difference 24 [95% CI −5.5 to 53.4] m, p = 0.11). NAC was safe and well-tolerated at the doses administered in this study.

Conclusion

In ambulatory RYR1-RM–affected individuals, we observed stable disease course, and corroborated preclinical reports of elevated oxidative stress and decreased physical endurance. NAC treatment did not decrease elevated oxidative stress, as measured by 15-F2t isoprostane.

Classification of evidence

This study provides Class I evidence that, for people with RYR1-RM, treatment with oral NAC does not decrease oxidative stress as measured by 15-F2t isoprostane.

Clinicaltrials.gov identifier

NCT02362425.

Cored in the act: the use of models to understand core myopathies

The core myopathies are a group of congenital myopathies with variable clinical expression - ranging from early-onset skeletal-muscle weakness to later-onset disease of variable severity - that are identified by characteristic 'core-like' lesions in myofibers and the presence of hypothonia and slowly or rather non-progressive muscle weakness. The genetic causes are diverse; central core disease is most often caused by mutations in ryanodine receptor 1 (RYR1), whereas multi-minicore disease is linked to pathogenic variants of several genes, including selenoprotein N (SELENON), RYR1 and titin (TTN). Understanding the mechanisms that drive core development and muscle weakness remains challenging due to the diversity of the excitation-contraction coupling (ECC) proteins involved and the differential effects of mutations across proteins. Because of this, the use of representative models expressing a mature ECC apparatus is crucial. Animal models have facilitated the identification of disease progression mechanisms for some mutations and have provided evidence to help explain genotype-phenotype correlations. However, many unanswered questions remain about the common and divergent pathological mechanisms that drive disease progression, and these mechanisms need to be understood in order to identify therapeutic targets. Several new transgenic animals have been described recently, expanding the spectrum of core myopathy models, including mice with patient-specific mutations. Furthermore, recent developments in 3D tissue engineering are expected to enable the study of core myopathy disease progression and the effects of potential therapeutic interventions in the context of human cells. In this Review, we summarize the current landscape of core myopathy models, and assess the hurdles and opportunities of future modeling strategies.

Translational medicine in neuromuscular disorders: from academia to industry

Although around half of US Food and Drug Administration (FDA)-approved drugs originate from discoveries made in academic research laboratories, the US National Institutes of Health (NIH) estimates that nearly 90% of therapies developed in preclinical stages never reach clinical trials. From those in clinical trials, only 10% obtain marketing approval. Despite the recent advances in our understanding and diagnosis of neuromuscular disease, and the development of rational therapies in clinical trials, these numbers have not changed dramatically over the past two decades. This article discusses the advantages and challenges for translational research initiated from academia, and the trend towards bridging the gap between discovery and translation to the clinic. A focus is made on recent advances in therapeutic development for neuromuscular disorders.

Summary: Academia-industry partnerships are important for therapeutic development. An improved understanding of the steps required for the translation of academic discoveries will be key for future clinical success in the neuromuscular field.

Early-Onset Myopathies: Clinical Findings, Prevalence of Subgroups and Diagnostic Approach in a Single Neuromuscular Referral Center in Germany

BACKGROUND

Early-onset myopathies are a heterogeneous group of neuromuscular diseases with broad clinical, genetic and histopathological overlap. The diagnostic approach has considerably changed since high throughput genetic methods (next generation sequencing, NGS) became available.

OBJECTIVE

We present diagnostic subgroups in a single neuromuscular referral center and describe an algorithm for the diagnostic work-up.

METHODS

The diagnostic approach of 98 index patients was retrospectively analysed. In 56 cases targeted sequencing of a known gene was performed, in 44 patients NGS was performed using large muscle specific panels, and in 12 individuals whole exome sequencing (WES) was undertaken. One patient was diagnosed via array CGH. Clinical features of all patients are provided.

RESULTS

The final diagnosis could be found in 63 out of 98 patients (64%) with molecular genetic analysis. In 55% targeted gene sequencing could establish the genetic diagnosis. However, this rate largely depended on the presence of distinct histological or clinical features. NGS (large myopathy-related panels and WES) revealed genetic diagnosis in 58.5% (52% and 67%, respectively). The genes detected by WES in our cohort of patients were all covered by the panels. Based on our findings we propose an algorithm for a practical diagnostic approach.Prevalences:MTM1- and LAMA2-patients are the two biggest subgroups, followed by SEPN1-, RYR1- and Collagen VI-related diseases. 31% of genetically confirmed cases represents a group with overlap between "congenital myopathies (CM)" and "congenital muscular dystrophies (CMD)". In 36% of the patients a specific genetic diagnosis could not be assigned.

CONCLUSIONS

A final diagnosis can be confirmed by high throughput genetic analysis in 58.5% of the cases, which is a higher rate than reported in the literature for muscle biopsy and should in many cases be considered as a first diagnostic tool. NGS cannot replace neuromuscular expertise and a close discussion with the geneticists on NGS is mandatory. Targeted candidate gene sequencing still plays a role in selected cases with highly suspicious clinical or histological features. There is a relevant clinical and genetic overlap between the entities CM and CMD.